We are grant-funded (NIH) to study the hormonal basis of maternal behavior in the rabbit. This study is being conducted at two sites; the Institute of Animal Behavior, Rutgers University, Newark, NJ and the Center for the Investigation of Animal Reproduction (CIRA in Spanish), Autonomous University of Tlaxcala, Tlaxcala, Mexico located in the town of Panotla. The grant allocates a portion of the funds as a sub-contract to Dr. Gabriela Gonzalez-Mariscal, Co-PI at CIRA and a portion to run parallel studies on the rat that will provide important information on the role of estrogen in stimulating maternal behavior that will be utilized in stimulating aspects of maternal behavior in the rabbit with estrogen. There are funds to send MBRS students to Mexico for periods up to 6 months to train them in studying rabbit maternal behavior after they have gained experience studying maternal behavior in the rat at the IAB. At Rutgers studies will investigate the changes in responsiveness to estrogen in stimulating maternal behavior during pregnancy. This is based on previous work in which females were shown to become more maternally responsive during pregnancy (following pregnancy termination by hysterectomy) showing shorter latencies for pup-stimulation of maternal behavior (sensitization). This change was correlated with increased concentration of nuclear estrogen receptors (NER) in the medial preoptic area (MPOA) during pregnancy and by the ability of estrogen to stimulate maternal behavior in hysterectomized-ovariectomized pregnant females. The basic design of our studies consists of hysterectomizing females at various times during pernancy (8, 10, 13, 16, 19 days) and also nonpregnant females and titrating the dose of hormone required to stimulate maternal behavior 48 hr later in at least 90 percent of females and to compare groups 3 days after hormone treatment. Bromocryptine will be injected in one group to block the release of prolactin to see if prolactin mediates the effect of estrogen in this preparation. Pregnant females left pregnant will also be studied as well as pseudopregnant females.